Field of the Invention
The present invention relates to a broadband antenna means.
Portable equipment, and then particularly hand-carried telephones, are often provided with helical antennas. Such antennas are convenient because they can be made more robust and take up less space than straight wire antennas for instance. However, they do not always give a satisfactory antenna performance.
One problem is that a helical antenna is short and can therefore easily be screened or obstructed by a user or by objects in its vicinity. The helical antenna is therefore often combined with an extendable straight wire antenna, which is not screened as easily and which also gives better antenna performance. For instance, the Patent Specifications U.S. Pat. No. 4,121,218, U.S. Pat. No. 5,204,687 and U.S. Pat. No. 4,868,576 describe different antenna means for portable equipment in which a helical antenna has been combined with an extendable straight wire antenna.
Another problem encountered with the use of helical antennas resides in their limited bandwidth. This renders conventional helical antennas unusable in those cases when an antenna function having a very broad band is required, for instance in the JDC mobile telephone system. In cases such as these, it would be desirable to provide an antenna means which has the positive properties of helical antennas coupled with a bandwidth which is much greater than the bandwidth of a conventional helical antenna.
It is, of course, conceivable to provide portable equipment with two separate helical antennas which have mutually different resonance frequencies. Such an antenna arrangement would be unnecessarily space consuming, however.